(1) Field of the Invention
The present invention relates to the field of rotorcraft fitted with at least one rotary wing rotor. The present invention relates more specifically to data processing methods and equipment adapted to a specific function of determining controlled variation in the speed at which at least one main rotor of substantially vertical axis of a rotorcraft is driven in rotation.
(2) Description of Related Art
Rotorcraft are rotary wing aircraft having at least one main rotor of substantially vertical axis that provides the rotorcraft with lift. If the rotorcraft is a helicopter, the main rotor provides not only lift, but also propulsion and/or control in pitching and in roll.
Compared with other powered aircraft, rotorcraft have the advantage of being able to hover and/or fly at low speed, including when close to the ground. As an indication, a rotorcraft is commonly said to be flying at low speeds when it is flying at less than about 50 knots (kt). Nevertheless, in cruising flight, rotorcraft are capable of flying at high speeds. As an indication, a rotorcraft is commonly said to be flying at high speed when flying at a speed lying in the range about 75 kt to 160 kt.
In this context, consideration should also be given to transitory flying speeds of the rotorcraft, i.e. flying speeds of the rotorcraft lying between low speeds and high speeds. Such transitory flying speeds of a rotorcraft are conventionally used for short periods only, between the rotorcraft flying at low speed and the rotorcraft flying at high speed.
In general, depending on the flying speed of a rotorcraft, its flight envelope is segmented into commonly accepted flying speed ranges of less than about 50 kt, transitory flying speeds, and greater than 75 kt. The values given for the ranges of rotorcraft flying speeds are given by way of indication and may be varied, in particular depending on the structure, the weight, and/or the performance of a particular rotorcraft, for example.
The lift of the rotorcraft is provided by the main rotor and it is controlled using flight controls operated by a pilot to vary the collective pitch of the blades making up the rotary wing of the main rotor. Such a pilot may be a human pilot or an autopilot. Driving rotation of the main rotor is considered as being a priority, given that it has the essential function of providing the rotorcraft with lift.
In the past, the rotary drive speed of the main rotor has long been established as being substantially constant. Nevertheless, proposals have been made to vary the drive speed of the main rotor voluntarily over a restricted range of speeds of rotation, in order to satisfy specific needs depending on various flight conditions of the rotorcraft.
For example, in order to improve the performance of a rotorcraft in a combat situation, it is known to vary the drive speed of the main rotor as a function of variation in the air speed of the rotorcraft. In this context, reference may be made to the publication “Enhanced energy maneuverability for attack helicopters using continuous variable rotor speed control” (C. G. Schaefer Jr.; F. H. Lutze Jr.); 47th Forum American Helicopter Society 1991, pp. 1293-1303.
Document US 2003/051461 (B. Certain) discloses regulating the speed of a rotorcraft engine used for driving rotors of the rotorcraft including a main rotor and a tail rotor of substantially horizontal axis, as a function of the position of the rudder control for varying the collective pitch of the blades of the tail rotor.
Nevertheless, it should be understood that multiple rotorcraft flight conditions can be taken into account in order to define a variable desired speed of rotation for the main rotor.
For example, concerning the performance of the rotorcraft, it is desirable for the pilot to have optimized ability to maneuver the rotorcraft, in particular at low flying speeds and/or in situations when the rotorcraft is close to the ground, including in the event of a possible failure of one of the rotorcraft engines used for driving rotation of the main rotor.
Also by way of example, it is also useful to take account of the noise produced by the rotorcraft in order to limit the sound nuisance that it generates while approaching or departing from a landing point.
In this context, reference may be made to Document U.S. Pat. No. 6,198,991 (Yamakawa et al.), which proposes reducing the sound nuisance generated by a rotorcraft approaching a landing point. In one possible implementation, it is proposed for that purpose to vary the speed of rotation of a main rotor of the rotorcraft in order to reduce the noise produced by the rotorcraft.
Also by way of example, reference may be made to Document WO 99/42360 (A. E. Karem), which proposes improving the performance obtained by a main rotor of a rotorcraft by causing its drive speed to vary depending on a load threshold supported by the blades of the main rotor.
Also by way of example, concerning the ambient outside medium in which the rotorcraft is flying, the physicochemical characteristics of the air have an influence on the lift of the rotorcraft for a given angle of incidence of the blades of the main rotor. The rotorcraft has various measuring and/or calculation instruments serving to identify the values of various parameters, such as the height of the rotorcraft above the ground, its pressure altitude, its density altitude, or the outside temperature, for example. On the basis of identifying the ambient outside medium or the flying speed of the rotorcraft, actions may be taken to modify the speed at which the main rotor is driven in rotation.
In this context, reference may be made to Document US 2007/118254 (G. W. Barnes, et al.), which proposes varying the speed of rotation of a main rotor of a rotorcraft depending on two values considered as being low and high under predefined threshold conditions for values of various parameters associated with previously-identified flight conditions of the rotorcraft.
Reference may also be made in this context to Document WO 2010/143051 (Agusta Spa et al.), which proposes varying the speed of rotation of a main rotor of a rotorcraft in compliance with a previously-established map depending on various flight conditions of the rotorcraft.